There has been conventionally known an air-fuel ratio control apparatus of an internal combustion engine when an output from an oxygen concentration sensor, which is disposed downstream of a catalyst converter, indicates rich and fuel is increased in quantity in the internal combustion engine, an air-fuel ratio is controlled to be a lean air-fuel ratio for a predetermined period of time, and thereafter, the air-fuel ratio is controlled to be returned to a theoretical air-fuel ratio (see Japanese Patent Application Laid-Open (JP-A) No. 63-117139). In addition, the prior art relating to the present invention is disclosed in JP-A Nos. 63-134835, 6-307271, 59-173533 or 2003-148202.
The capacity of a catalyst in an exhaust gas purifying system mounted on a vehicle or the like has been increased in order to cope with the reinforcement of an exhaust emission control. Therefore, there is a possibility that a conventional air-fuel ratio control cannot suppress generation of a catalyst exhaust gas odor (specifically, an odor of hydrogen sulfide (H2S)), because a quantity of oxygen occluded in an exhaust gas catalyst is small till deceleration or stoppage of the vehicle after the internal combustion engine is operated in an increased quantity of fuel. In order to occlude the oxygen in quantity enough to suppress the generation of the catalyst exhaust gas odor with respect to the catalyst till the deceleration or stoppage of the internal combustion engine, for example, the air-fuel ratio is largely changed onto a lean side by cutting the fuel or the internal combustion engine is operated in a lean air-fuel ratio for a long period of time. However, there may be a problem of a misfire in the internal combustion engine in the case that the air-fuel ratio is largely changed onto the lean side, while there may be a problem of degradation of exhaust emission due to an increase in NOx generation quantity during operation in a lean air-fuel ratio for a long period of time. Additionally, the degradation of the catalyst is prompted in the atmosphere in which the oxygen is excessively present at a high temperature.